Calcium carbonate occurs naturally as limestone rock, which is generally defined as rock containing greater than 50 percent by weight calcium carbonate. This natural calcium carbonate is generally mined or quarried to produce a calcium carbonate product having designated size and purity characteristics.
Typical processing of limestone rock includes crushing, grinding and classification. Also, limestone rock is a raw material for precipitated grades of calcium carbonate. Precipitated calcium carbonate is typically manufactured by calcining limestone, thereby producing carbon dioxide and calcium oxide. The calcium oxide is hydrated to form calcium hydroxide, which is then carbonated to form calcium carbonate. After precipitation, the calcium carbonate may be further processed by screening and drying.
Other processes for the manufacture of calcium carbonate, such as the use of sodium carbonate to precipitate calcium carbonate and the calcium chloride process, are known in the art.
Natural calcium carbonate is widely used as a white pigment. Synthetic, or precipitated calcium carbonates are used in the manufacture of paper, paints, plastics, inks, rubber, adhesives, caulks, and sealants.
Irrespective of the processing technique used to manufacture calcium carbonate, the settling of fine calcium carbonate particles from slurries or suspensions remains a problem. The instant invention, which relates to compositions and a method for settling fine calcium carbonate particles from suspensions or solutions, thus constitutes a notable advance in the art.
As used herein, the term calcium carbonate includes both the naturally occurring and synthetic, or precipitated, calcium carbonate. The term also encompasses all crystalline structures of calcium carbonate, including but not limited to calcite and aragonite.
Settling, as used herein, is defined as the separation of a slurry or suspension into components having greater and lesser concentrations of the solid or solids than does the original slurry or suspension. As such, the terms settling, sedimentation, and separation are synonomous and interchangeable.
Suspensions and slurries are defined as liquid-solids systems wherein the solids are dispersed or entrained in a liquid. For example, slurries or suspensions may comprise water as the liquid and calcium carbonate as the solid.
Calcium carbonate particles less than 2 microns in size are particularly difficult to settle from a calcium carbonate-liquid system. The inventors have found that, by adding specified compositions of polyelectrolytes and surfactants to these systems or suspensions, at designated dosages, these fine calcium carbonate particles rapidly settle, thereby producing a clear supernatant liquid and a more concentrated solid phase. This improved separation increases calcium carbonate yields, while minimizing water treatment costs.
Though settling aids such as Accurac 161, a nonionic emulsion commercially available from American Cyanamid Company, have been used to settle calcium carbonate particles from slurries or suspensions, no settling aids known to the inventor have the ability to settle fine calcium carbonate particles as do the instant compositions. Additionally, the instant compositions produce a soft, flowable cake while improving overall settling rates.